Can you easily export a .blend to a previous version of Blender?
TLDR: Is it possible? Yes.  Is it easy? It's not one-click.
Blender will attempt to open any .blend from any version, but the success rate is not that hot.  With the advent of collections instead of scenes, the scene will open empty.  So going from 2.9 to 2.79 directly will not fly.
However, if you are willing to jump through some hoops, you can extract things of interest out of those files with the Link/Append functions.
Just make sure to choose the right object. You will also need to make sure that modifiers and nodes that were added since were not used in the object you are trying to back-port.
I successfully yanked one of my characters out of a 2.9 version file and got it running on 2.79, but there was some fiddling I had to do and I wasn't doing anything too special rigging wise. Also remember that you must use Cycles in 2.79, Blender render materials are not compatible with Cycles/EEVEE.